EP Patent No. 1 422 436 discloses a balance spring formed of silicon and coated with silicon dioxide so as to make the thermal coefficient substantially zero around COSC (Swiss Official Chronometer Testing Institute) certification process temperatures, i.e. between +8 and +38° C. Likewise, WO Patent No. 2008-043727 discloses a MEMS resonator which has similar properties of low variation from its Young's modulus within the same temperature range.
However, the frequency variation in the above disclosures can require complex corrections depending upon the application. For example, for electronic quartz watches to be able to be COSC certified, an electronic correction has to be carried out based on a temperature measurement.